Operation of reverse osmosis (RO) in cyclic batch mode can in principle provide both high energy efficiency and high recovery. However, one factor that causes the performance to be less than ideal is longitudinal dispersion in the RO module. At the end of the batch pressurisation phase it is necessary to purge and then refill the module. During the purge and refill phases, dispersion causes undesirable mixing of concentrated brine with less concentrated feed water, therefore increasing the salt concentration and energy usage in the subsequent pressurisation phase of the cycle. In this study, we quantify the significance of dispersion through theory and experiment. We provide an analysis that relates the energy efficiency of the batch operation to the amount of dispersion. With the help of a model based on the analysis by Taylor, dispersion is quantified according to flow rate. The model is confirmed by experiments with two types of proprietary spiral wound RO modules, using sodium chloride (NaCl) solutions of concentration 1000 to 20,000 ppm. In practice the typical energy usage increases by 4% to 5.5% compared to the ideal case of zero dispersion.